Pneumatic conveying by pressure or vacuum are techniques employed to transport particulate materials along a pipeline. These techniques are typically employed to transport materials over distances typically in the range from 10 m to 500 m, and in some cases even further. Pneumatic conveying avoids the need for the use of conveyor belts or the like, which can be bulky and costly to maintain.
Pneumatic conveying techniques are particularly useful where material has to be transported along a complex path, or to multiple delivery points. These techniques also ensure that the particulate material can be entirely contained within a pipeline, which may avoid the need to deal with dust from or contamination of the material along the path of the conveying pipeline.
Dense phase positive pressure or vacuum pneumatic conveying is often used to transport dense phase particulates that are not suitable for conveying by suspension in a gas flow, such as materials prone to aggregate or coagulate, or particularly abrasive or friable materials. In dense phase pneumatic conveying, such materials are conveyed along a pipeline at relatively low velocities, often in a series of “material slugs”. By keeping the transport velocity low, both pipeline wear and energy consumption is reduced.
A conventional pressurised dense phase pneumatic conveying system 1 is shown in FIG. 1(a). A particulate material 3 is delivered from a hopper 5, into a pressure vessel 7 (commonly referred to as a “transporter”) via a material shut-off valve 9. The pressure vessel is pressurised with compressed air, delivered for example from a compressor 11a via a control valve 13. The pressurised air in the pressure vessel 7 expands into the conveying pipeline 17 and the air flow propels particulate material 15 along the pipeline to a delivery point (e.g. a second hopper, 19).
Dense phase vacuum conveying uses a similar principle. As shown in FIG. 1(b), instead of the pressure differential between the inlet and outlet of the conveying pipeline being achieved by pressurising the transporter, in vacuum conveying the pipeline inlet is at ambient pressure and the pressure at the outlet (for example in the second hopper 19) is reduced, by a vacuum pump 11b. 
Some materials are unsuitable for dense phase conveying without additional assistive technologies being applied directly to the conveying pipeline. For example, some materials have low permeability to the motive gas flow. When combined with high friction between the particulate material and the inner wall of the pipeline, movement of the material can become erratic and unpredictable, which may lead to variable conveying rate performance and/or pipeline blockages.
To seek to address these issues, it is known to inject compressed air through a plurality of delivery points positioned at intervals along the pipeline length. However, this approach often requires higher volumes and/or pressures of compressed air. This additional consumption arises because air is unnecessarily injected at some points along the pipeline. In turn, the additional air flow increases the particulate material velocity along the pipeline, which may lead to increased pipeline wear or damage caused by contact with the particulate material.
One approach to minimizing air consumption has been to provide injectors along a pipeline with pressure transducers, and to inject compressed air via non-return valves only at specific injectors, in response to pressure conditions in the pipeline. Examples of such systems are described in U.S. Pat. Nos. 4,515,503, 5,584,612 and GB2085388. Systems which trigger gas injection above an absolute threshold pipeline pressure can be difficult to implement, because the required threshold pressure decreases along a pipeline (requiring individual adjustment) and can be dependent on the type of material being conveyed. GB2085388, for example, teaches that reference values for the pressure above which compressed air is injected, are selected for each type of material being conveyed.
These systems have several further drawbacks. Since they work on the principle of detecting an increased pipeline pressure characteristic of a material plug, and “pushing” the plug along the pipeline by injection additional air, the tendency is for each injector in turn to be switched on and remain on as the plug progresses down the pipeline, leading to wastage of air. In addition, injectors and non-return valves are prone to blockage when not in use, from contaminated backflow from the conveying pipeline.
Systems such as those described above are also prone under some circumstances to exacerbate problems by injecting gas upstream of a material plug, thereby compacting it.
U.S. Pat. No. 4,861,200 describes a system in which the pressure differential between a reference line and a conveying pipeline is measured (Δpn) at each of a series of groups of injectors arrayed along the pipeline. A larger than expected pressure drop along the pipeline is indicative of a blockage and so the pressure differential switches are arranged such that, where Δpn exceeds Δpn+1 at the adjacent groups of injectors downstream by a predetermined amount, compressed air is injected through the upstream injectors. The comparison to a reference value means that each pressure difference Δp must be calibrated to a “idealised” pressure drop along the pipeline, which is once again material-specific.
Conveying velocity and pressure may also be limited by allowing excess conveying gas pressure and volume to “bypass” a material plug, for example via an internal fluted pipeline or external pressure release valve-controlled bypass loops. Again, however, the “bypass” arrangements may be prone to blockage and wear. In the case of internal bypass pipelines, repair or replacement can be particularly difficult and costly.
Accordingly, there remains a need to improvements to methods and apparatus for dense phase pneumatic conveying.